The present invention relates to a hollow waveguide directional coupler having two hollow waveguide sections connected to each other by a plurality of coupling openings formed in a wall extending between the hollow waveguide sections.
Such directional couplers are generally known. Directional couplers of this type having two coupling openings are described e.g. in Meinke-Gundlach, Taschenbuch der Hochfrequenztechnik, 5. Auflage, Springer 1992, Chapter L7.4. Such directional couplers have good directivity for wavelengths λg in a range of four times the distance between the coupling openings, where λg is the wavelength of the nominal centre frequency of the operating frequency range of the directional coupler. The directivity is based on the fact that the fields coupled at the two openings from the transit hollow waveguide section to the driven hollow waveguide section propagate in both directions in the driven hollow waveguide, and that the fields that propagate in the same direction as in the transit waveguide interfere constructively, whereas, in the opposite direction, they have a phase difference of λ/2 and thus cancel each other.
Hollow waveguide directional couplers having a plurality of coupling openings are described e.g. in R. Levy “Analysis and Synthesis of Waveguide Multi-aperture Directional Couplers”, IEEE Trans. on Microwave Theory and Tech., Vol. MTT-16, No. 12, December 1968 and in H. Schmiedel et al. “Field Theory Design of Rectangular Waveguide Multiple-Slot Narrow-Wall Couplers”, IEEE Trans. on Microwave Theory and Tech., Vol. MTT-34, No. 7, July 1986.
The dimensions of these coupling openings and the wall sections between them are obviously proportional to the RF wavelengths with which they are to be used. Applications in the millimeter-wavelength range require dimensions of the coupling openings and the wall portions which are difficult to manufacture with conventional manufacturing techniques such as edging and machining. In particular when strong couplings are required, a high number of coupling openings is necessary, because the cross section area of a single coupling opening is limited by the requirement that a wall portion separating it from an adjacent coupling opening must be strong enough to withstand the loads to which it is exposed during manufacture.
It is readily apparent that the effort for manufacturing such a directional coupler increases with the number of coupling openings that must be manufactured. The effort for the manufacture of the directional coupler does not only increase linearly with the number of required coupling openings or, what is more or less equivalent, with the length of the coupling section, but exponentially. The reason for this is the non-vanishing probability that during manufacture a wall portion becomes damaged and the complete directional coupler is thus spoiled. The higher the number of existing wall portions, the less is the probability that all of them will survive the manufacture undamaged.